Methods for preparing diol

ABSTRACT

Provided is a method for preparing a diol. In the method, a saccharide and hydrogen as raw materials are contacted with a catalyst in water to prepare the diol. The employed catalyst is a composite catalyst comprised of a main catalyst and a cocatalyst, wherein the main catalyst is a water-insoluble acid-resistant alloy; and the cocatalyst is a soluble tungstate and/or soluble tungsten compound. The method uses an acid-resistant, inexpensive and stable alloy needless of a support as a main catalyst, and can guarantee a high yield of the diol in the case where the production cost is relatively low.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application under 35 U.S.C. § 371of International Application No. PCT/CN2015/090321, filed on Sep. 23,2015, which claims priority to Chinese Patent Application No.201410512704.7, filed Sep. 28, 2014. The complete disclosure of each ofthe above-identified applications is fully incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a method for preparing a diol.

BACKGROUND ART

Ethylene glycol, as an important monomer for bottle-grade polyester andfiber-grade polyester, has a very large application market. Propyleneglycol may be widely used in the food, pharmaceutical and cosmeticsindustries. For a long time now, diols such as ethylene glycol andpropylene glycol have been mainly produced using petroleum-based olefinsas starting materials, by methods such as the two step method ofoxidation and hydration. However, as petroleum resources are graduallydepleted, the utilization of renewable starting materials to preparediols has huge commercial prospects.

A process for preparing ethylene glycol by one-step catalytichydrocracking, using soluble sugar as a starting material, has beendisclosed in the prior art. The process is simple and the startingmaterial is abundant, so the process has prospects for large-scalecommercial production. However, the process has various deficiencies.For example, starting material sugars are of low concentration (e.g. WO2013015955 A, CN 102020531 A), a precious metal or a combination of aprecious metal and a cheap metal is used as a catalyst (e.g. U.S. Pat.No. 4,496,780 A, CN 102643165 A, CN 103420797 A), the ethylene glycolyield is low (e.g. U.S. Pat. No. 4,496,780 A, CN 102731259 A, CN103420787 A, CN 101735014 A, CN 101613253 A, CN 103667365 A), etc., sothat the ethylene glycol production cost is too high, the catalystactivity is unstable, and continuous production is not possible.

Research has found that in a process for preparing a diol by one-stepcatalytic hydrocracking of soluble sugar, starting material sugar veryreadily undergoes side reactions such as hydrolysis underhigh-temperature aqueous phase conditions, producing small-moleculesubstances such as acetic acid, lactic acid, formic acid, furan,aldehydes and alcohols etc., in turn leading to an increase in theacidity of the system (Sevilla M, Fuertes A B. Chemical and structuralproperties of carbonaceous products obtained by hydrothermalcarbonization of saccharides. Chemistry-A European Journal. 2009,15(16): 4195-4203.). At the same time, polymers formed by furthercondensation polymerization of the aldehydes and alcohols etc. producedwill block catalyst pores, and this will lower the catalyst's catalyticactivity, service life and selectivity as well as the long-termoperational stability of the reaction system; the result is that theprocess has poor economic feasibility, and cannot be used forlarge-scale, continuous production. At the same time, the production ofby-products also leads to a drop in the diol yield. It is disclosed inexisting patent applications that 40-60% of the starting material sugarwill undergo a hydrolysis side reaction (U.S. Pat. No. 5,107,018, CN101781167 A, CN 101781171 A, CN 101781166 A).

When the concentration of starting material sugar is high, underhigh-temperature aqueous phase conditions, in the first place it moreeasily undergoes polymerization and thereby blocks catalyst channels,leading to a shortening of the life of the catalyst, and an increase inthe production cost of diols, and for this reason the requirements forcatalytic activity of the catalyst are higher in order that the startingmaterial sugar is hydrocracked before it undergoes polymerization. Inthe second place, acid of higher concentration is produced more easily,and for this reason the requirements for acidity resistance of thecatalyst are higher. Therefore, in most existing patent applications,sugar of low concentration is used as a starting material. For example,CN 102190562 A and CN 101735014 A employ a composite catalyst formed ofa tungsten compound and an active component, and a monosaccharide of 1%glucose etc. dissolved in water as a starting material, and the ethyleneglycol yield is 30-45%. CN 103420796 A employs a composite catalyst ofRu/C and tungstic acid, and a monosaccharide of 1% glucose etc.dissolved in water as a starting material; the catalyst is recycledintermittently, and the ethylene glycol yield is 52-57%. CN 102731258 Aemploys an Ni—W₂C/AC supported catalyst, and 18% glucose as a startingmaterial, and the diol yield is 50-60%, wherein the ethylene glycolyield is 55%. These applications have good ethylene glycol yields, buthave the following shortcomings due to the low concentration of startingmaterial sugar used: Firstly, the glucose concentration is 1-18%, so thereaction system contains a large amount of water; the boiling point ofethylene glycol is higher than that of water, being 197.3° C., so whenseparation by rectification is performed, the system must first consumea large amount of heat in distilling off the water, leading to highseparation costs, so production is not economical. Secondly, theseapplications all use activated carbon as a support, but activated carbonreadily undergoes a hydrogenation reaction under high-temperatureconditions in the presence of hydrogen, thereby being methanized(US2002/0169344). The existing patent application CN 102643165 A hasdisclosed the use of 40-60% glucose as a starting material, and the diolyield is 50-60%; however, that application uses Ru/C with an activatedcarbon support as the catalyst; using a precious metal as a catalystwill make the production cost high, there is a risk of the activatedcarbon being methanized, and the continuous operational stability ofthat application is unknown.

In processes for preparing diols by one-step catalytic hydrocracking ofsoluble sugar, commonly used catalysts include cheap metals (such asnickel) and precious metals. In the case where a nickel-containingcatalyst is used as a catalyst, when the acidity of the reaction systemincreases due to starting material sugar undergoing a hydrolysis sidereaction, the nickel will undergo a reaction, releasing hydrogen andproducing nickel ions Ni²⁺, so that the nickel-containing catalystslowly dissolves, losing its hydrogenating activity. It has beenreported in the literature that the reaction system pH may be regulatedat 7 or higher to maintain the stability of the nickel-containingcatalyst (CN 103667365 A). Under high pH conditions, the propyleneglycol yield will increase significantly while the ethylene glycol yieldwill decrease significantly (U.S. Pat. No. 5,107,018, CN 101781167 A, CN101781171 A, CN 101781166 A); at the same time, acids produced in thehydrolysis side reaction such as formic acid, acetic acid and lacticacid will increase, and the total diol yield will fall (CN 101544537 A).Li Yan et al. have found that under acidic conditions of pH<5, thestarting material sugar is in a more stable state, and essentially doesnot undergo a hydrolysis side reaction (Li Yan, Shen Canqiu et al.,Research on the decomposition mechanism of sucrose in impure sugarsolutions, China Beet and Sugar, 1996(2): 11-16); thus, the diol yieldof a sugar hydrocracking system can be increased if the latter operatesunder acidic conditions. When a precious metal such as Ru or Pt is usedas a catalyst, it can exist stably under low pH conditions, but willsignificantly increase the diol production cost. To reduce the amount ofprecious metal used and increase its catalytic activity, people selectsupports with a high specific surface area to fix and disperse it. Anexample of commonly used supports is inorganic oxides such as alumina,silica and magnesia, which are unstable under acidic conditions, andreadily undergo a neutralization reaction and dissolve in the reactionsystem, leading to a fall in the diol yield (CN 103159587 A); anotherexample is activated carbon (CN 103420796 A, CN 102643165 A, CN102731258 A, CN 101613253 A), which readily undergoes a hydrogenationreaction and is methanized under high-temperature conditions in thepresence of hydrogen.

In summary, a new diol preparation method is needed. A diol is producedat low cost through the use of an acid-resistant, cheap and stablecatalyst.

Content of the Present Invention

The object of the present invention is to provide a method for preparinga diol. The present invention uses an acid-resistant, cheap and stablealloy, which does not need a support, as a main catalyst to prepare adiol.

The present invention employs the following technical solution:

A method for preparing a diol, which method uses a sugar and hydrogen asstarting materials, which are brought into contact with a catalyst inwater to prepare a diol; the catalyst used is a composite catalyst,consisting of a main catalyst and a cocatalyst,

wherein

the main catalyst is a water-insoluble acid-resistant alloy;

the cocatalyst is a soluble tungstic acid salt and/or an insolubletungsten compound.

Preferably, the diol is ethylene glycol.

The present invention uses an acid-resistant, cheap and stable alloy,which does not need a support and is insoluble in water, as a maincatalyst, which is used in cooperation with a cocatalyst of a solubletungstic acid salt and/or an insoluble tungsten compound, to catalysesugar as a composite catalyst to obtain a diol; the yield of diol, inparticular ethylene glycol, can be ensured at a low production cost. Thewater-insoluble, acid-resistant alloy of the present invention is stableunder acidic conditions, and there is no need to add an alkali to thereaction system to neutralize acid formed by hydrolysis of sugar. Whenthe method of the present invention is used in continuous industrialproduction, the use of such an acid-resistant alloy main catalyst isespecially important for the long-term, stable operation of the systemand for control of production costs.

Preferably, when ethylene glycol is prepared by the method describedabove, the reaction system pH is 1-7; more preferably, the reactionsystem pH is 3-6. By keeping the system pH<7, not only can a hydrolysisside reaction of starting material sugar during the reaction be avoided,thereby reducing the amount of starting material sugar consumed inethylene glycol production, but also the service life of the catalyst isensured, so the cost of using the catalyst can be reduced, the stabilityof long-term continuous operation of the reaction system can be ensured;at the same time, the ethylene glycol yield is high, and the output oforganic acids and polymers is low. If acids produced in the course ofthe reaction are not enough to maintain a low pH, inorganic acids ororganic acids such as lactic acid, formic acid and acetic acid may beadded to the system to regulate the pH of the reaction system.Generally, organic acid or inorganic acid is added together withstarting material sugar.

Preferably, the sugar is selected from one or more of five-carbonmonosaccharides, disaccharides and oligosaccharides, six-carbonmonosaccharides, disaccharides and oligosaccharides, soluble five-carbonpolysaccharides, and soluble six-carbon polysaccharides. Originalsources of the starting material sugar include but are not limited tosugar-based substances such as beet and sugarcane, starch-basedsubstances such as maize, wheat, barley and cassava,lignocellulose-based substances such as maize straw, corn cobs, wheatstraw, sugarcane dregs and timber, cellulosic industrial residue such ascorn cob dregs, or polysaccharide substances including algae, etc. Inthis text, soluble five-carbon polysaccharides and soluble six-carbonpolysaccharides are five-carbon polysaccharides and six-carbonpolysaccharides which can dissolve under the reaction conditions of thepresent invention, not just five-carbon polysaccharides and six-carbonpolysaccharides which can dissolve at room temperature.

Preferably, the sugar reacts with hydrogen in the form of an aqueoussugar solution (abbreviated as sugar solution), and the aqueous sugarsolution has a concentration of 5-60 wt %, more preferably 20-50 wt %.In a continuous operation, the sugar solution may be fed continuously bymeans of a delivery pump. In the present invention, a suitable catalystis selected so that the restriction imposed on starting material sugarconcentration by the reaction system is smaller; sugar solution of highconcentration may be used as a starting material, and this willsignificantly reduce the production cost of diol, in particular ethyleneglycol, thereby realizing large-scale and economical diol production.

Furthermore, the acid-resistant alloy comprises nickel, one or more rareearth elements, tin and aluminum; the parts by weight of the componentsare preferably 10-90 parts, 1-5 parts, 1-60 parts and 5-9 partsrespectively.

In this text, rare earth elements is a collective term for 17 chemicalelements, with atomic numbers 21, 39 and 57-71, in group IIIB of theperiodic table, including lanthanum (La), cerium (Ce) and samarium (Sm)etc.

More preferably, the acid-resistant alloy comprises nickel, one or morerare earth elements, tin, aluminum and tungsten; the parts by weight ofthe components are preferably 10-90 parts, 1-5 parts, 1-60 parts, 5-9parts and 1-90 parts respectively.

Further preferably, the acid-resistant alloy comprises nickel, one ormore rare earth elements, tin, aluminum, tungsten and molybdenum; theparts by weight of the components are preferably 10-90 parts, 1-5 parts,1-60 parts, 5-9 parts, 1-90 parts and 0.5-20 parts respectively.

Most preferably, the acid-resistant alloy comprises nickel, one or morerare earth elements, tin, aluminum, tungsten, molybdenum, and boron orphosphorus; the parts by weight of the components are preferably 10-90parts, 1-5 parts, 1-60 parts, 5-9 parts, 1-parts, 0.5-20 parts and0.01-5 parts respectively.

Preferably, the soluble tungstic acid salt is one or more of ammoniumtungstate, sodium tungstate and sodium phosphotungstate; the insolubletungsten compound is tungsten trioxide and/or tungstic acid.

The main catalyst is mixed with water and then added to a reactor.

Preferably, the amount of the main catalyst used is 0.01-10 times theamount of sugar fed per hour.

Preferably, the reaction is in continuous mode.

Preferably, the amount of main catalyst added is: 0.01-5 kg of maincatalyst added per 1000 kg of sugar fed. The addition of catalyst may berealized by discharging a portion of old catalyst through a catalystoutput valve (generally at the bottom of the reactor), then adding thesame amount of new catalyst through a catalyst feed valve (generally atthe bottom of the reactor.

The soluble cocatalyst may be first added to sugar solution, then thesemay be added to the reactor together. Preferably, the amount of thesoluble cocatalyst used is 0.01-5 wt % of the aqueous sugar solution,more preferably 0.01-2 wt %, and most preferably 0.01-1 wt %.

The insoluble cocatalyst may be added to the reactor together with themain catalyst. Preferably, the amount of the insoluble cocatalyst usedis 0.5-50 wt % of the main catalyst, more preferably 5-20 wt %.

Preferably, the reaction system has a reaction pressure of 5-12 MPa, areaction temperature of 150-260° C., and a reaction time 10 min.

More preferably, the reaction system has a reaction pressure of 6-10MPa, a reaction temperature of 180-250° C., and a reaction time of 0.5-3h. The reaction time is most preferably 0.5-2 hours.

Preferably, the reaction takes place in a slurry bed reactor. To ensurethat the reaction proceeds smoothly, the total volume of reaction liquidformed does not exceed 80% of the reactor volume.

Preferably, a filter is provided in the slurry bed reactor, for causingan insoluble portion of the catalyst to be retained in the reactor, andnot carried away by gas and reaction liquid flowing out through thefilter.

Before the reaction begins, main catalyst is added to the slurry bedreactor, and hydrogen and sugar solution are added to the reactor at thesame time using respective pumps, and a reaction takes place; theaddition of sugar and main catalyst is in a continuous flow state, andreaction liquid flows out of the reactor continuously. Regarding thecocatalyst, when it is a soluble tungsten compound, it is added to thereactor together with sugar solution; when it is an insoluble tungstencompound, it is added to the reactor at the same time as the maincatalyst. A filter is installed in the reactor. The filter can interceptcatalyst, but gas and reaction liquid will flow out continuously throughthe filter and enter a condenser to undergo gas/liquid separation. Crudehydrogen undergoes purification to remove CO, CO₂ and CH₄ etc., andbecomes purified hydrogen again, returning to the reactor. Effluentflowing out of the condenser enters a separation system, and isseparated to obtain water, ethylene glycol, propylene glycol, butyleneglycol, glycerol, sorbitol and cocatalyst, etc. Products such asethylene glycol, propylene glycol and butylene glycol may be obtained bypurification using existing technology (e.g. rectification). Water,sorbitol, glycerol and cocatalyst that is already dissolved in thereaction system are returned to the reactor to react in a cycle.

The beneficial effects of the present invention are as follows:

1. The catalyst of the present invention is cheap, stable, and does notneed a support.

2. The present invention can use a sugar solution of high concentrationas a starting material, so the production cost of diols, in particularethylene glycol, is low.

3. The method of the present invention gives a high ethylene glycolyield.

DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a schematic flow chart of the method of the present invention.

FIG. 2 is a graph of the variation of ethylene glycol yield with time inembodiment 2.

PARTICULAR EMBODIMENTS

The present invention is explained further below in conjunction with theaccompanying drawings and embodiments.

FIG. 1 is a schematic flow chart of the method of the present invention.

Embodiment 1

Preparation of Acid-Resistant Alloy Main Catalyst:

With regard to the acid-resistant alloy main catalyst of the presentinvention, an active metal powder with a high specific surface area canbe prepared directly by chemical reduction or electrolytic deposition;alternatively, a metal alloy is formed by smelting, then metal powder isformed by mechanical pulverizing or atomizing, etc., and finally, anactive metal powder is formed by a conventional Raney nickel catalystactivation method. For example, in parts by weight, 10-90 parts, 1-5parts, 1-60 parts, 5-9 parts, 1-90 parts, 0.5-20 parts and 0.01-5 partsof nickel, rare earth element, tin, aluminum, tungsten, molybdenum, andboron or phosphorus respectively are added to a smelting furnace; thetemperature is raised to 1500-2000° C., then the temperature is lowered,and after thorough mechanical stirring to achieve uniformity, thefurnace is emptied, to obtain he metal alloy. A hammer grinder is usedto pulverize the metal alloy into metal powder, which is then immersedfor 1-2 hours in a 20 wt %-25 wt % aqueous sodium hydroxide solution at70-95° C., to form an active metal powder with a high specific surfacearea.

An acid-resistant alloy main catalyst Ni80La1Sn30Al5 (indicating thatthe composition of the acid-resistant alloy is 80 parts Ni+1 part La+30parts Sn+5 parts Al, likewise below), an acid-resistant alloy maincatalyst Ni10Sm5Sn3Al9W70Mo5, an acid-resistant alloy main catalystNi70Ce1Sn50Al7W5Mo1B5, an acid-resistant alloy main catalystNi90Ce3Sn60Al9W20Mo5B1, an acid-resistant alloy main catalystNi10Sm5Sn10Al9W90, an acid-resistant alloy main catalystNi90Ce3Sn60Al9W20Mo20P0.01, and an acid-resistant alloy main catalystNi80La1Ce0.5Sn30Al5 are prepared separately.

Embodiment 2

6 L of water and 1000 g of acid-resistant alloy main catalystNi80La1Sn30Al5 are added to a 10 L reaction kettle while stirring. Thereaction kettle is sealed, hydrogen is passed in for 5 hours at 1000 L/hat atmospheric pressure to replace air in the reaction kettle, then thehydrogen pressure is raised to 10 MPa, and hydrogen is passed in for afurther 5 hours, the reaction kettle temperature is raised to 250° C.,and continuous feeding begins. The feed composition is: 50 wt % glucose,2 wt % sodium tungstate, 48 wt % water, and the density of the sugarsolution is about 1.23 g/cm³; the feed rate is 3 L/h. The residence timeof sugar in the reaction kettle is 2 hours. Acetic acid is added to thereaction kettle such that the reaction system pH is 3.5. Reaction liquidand hydrogen after the reaction flow out of the reaction kettle througha filter into a condensing tank; the output speed of reaction liquid isL/h, and reaction liquid is discharged from the bottom of the condensingtank after cooling, to give effluent. The effluent enters arectification separation system, and water, ethylene glycol, propyleneglycol, glycerol and sorbitol and sodium tungstate are respectivelyobtained, wherein heavy components that are not distilled out, includingglycerol and sorbitol and sodium tungstate, are returned to the reactionsystem to react in a cycle. A sample is taken at the bottom of thecondensing tank, and the composition thereof is detected by highperformance liquid chromatography.

A conventional technique may be used for the high performance liquidchromatography detection. The present invention provides the followingexperimental parameters for reference:

Instrument: Waters 515 HPLC Pump;

Detector: Water 2414 Refractive Index Detector;

Chromatography column: 300 mm×7.8 mm, Aminex HPX-87H ion exchangecolumn;

Mobile phase: 5 mmol/L sulphuric acid solution;

Mobile phase flow rate: 0.6 ml/min;

Column temperature: 60° C.;

Detector temperature: 40° C.

Results: the glucose conversion rate is 100%; the diol yield is 77%,wherein the ethylene glycol yield is 71%, the propylene glycol yield is7%, and the butylene glycol yield is 3%; the methanol and ethanol yieldis 5%, and other yields are 14%.

FIG. 2 is a graph of the variation of ethylene glycol yield withreaction system operation time. It can be seen from the figure that theethylene glycol yield is substantially maintained at about 70%. Thisindicates that the composite catalyst of the present invention canensure that the ethylene glycol yield is still stable after 500 hours ofcontinuous operation of the reaction system.

When the reaction system pH is changed to 9, the results are: theglucose conversion rate is 100%; the diol yield is 68%, wherein theethylene glycol yield is 38%, the propylene glycol yield is 27%, and thebutylene glycol yield is 3%; the methanol and ethanol yield is 5%, andother yields are 27%.

Embodiment 3

The acid-resistant alloy main catalyst is Ni10Sm5Sn3Al9W70Mo5, and theamount added is 5000 g.

The feed composition is: 15 wt % glucose, 0.01 wt % ammonium tungstate,84.9 wt % water, and the density of the sugar solution is about 1.06g/cm³.

Reaction system pH=6.

Other operating conditions are the same as in embodiment 2.

Results: the glucose conversion rate is 100%; the diol yield is 66%,wherein the ethylene glycol yield is 61%, the propylene glycol yield is3%, and the butylene glycol yield is 2%; the methanol and ethanol yieldis 9%, and other yields are 25%.

Embodiment 4

The acid-resistant alloy main catalyst is Ni70Ce1Sn50Al7W5Mo1B5, and theamount added is 500 g.

The amount of tungsten trioxide added is 100 g.

The feed composition is: 40 wt % glucose, 60 wt % water, and the densityof the sugar solution is about 1.18 g/cm³.

Reaction system pH=4.2.

Other operating conditions are the same as in embodiment 2.

Results: the glucose conversion rate is 100%; the diol yield is 70%,wherein the ethylene glycol yield is 67%, the propylene glycol yield is2%, and the butylene glycol yield is 1%; the methanol and ethanol yieldis 9%, and other yields are 21%.

Embodiment 5

The acid-resistant alloy main catalyst is Ni90Ce3Sn60Al9W20Mo5B1, andthe amount added is 1000 g.

The feed composition is: 15 wt % xylose, 40 wt % glucose, wt % maltose,1 wt % maltotriose, 1 wt % sodium phosphotungstate, 42 wt % water, andthe density of the sugar solution is about 1.22 g/cm³.

Reaction system pH=4.8.

Other operating conditions are the same as in embodiment 2.

Results: the conversion rate of xylose, glucose, maltose and maltotrioseis 100%; the diol yield is 75%, wherein the ethylene glycol yield is60%, the propylene glycol yield is 11%, and the butylene glycol yield is4%; the methanol and ethanol yield is 7%, and other yields are 18%.After 500 hours of catalyst operation, the ethylene glycol yield isstill stable.

Embodiment 6

The acid-resistant alloy main catalyst is Ni90Ce3Sn60Al9W20Mo5B1, andthe amount added is 5000 g.

The feed composition is: 50 wt % xylose, 0.1 wt % sodium tungstate, 49.9wt % water, and the density of the sugar solution is about 1.21 g/cm³.

Reaction system pH=4.8.

Other operating conditions are the same as in embodiment 2.

Results: the conversion rate of xylose is 100%; the diol yield is 67%,wherein the ethylene glycol yield is 49%, the propylene glycol yield is16%, and the butylene glycol yield is 2%; the methanol and ethanol yieldis 12%, and other yields are 21%. After 500 hours of catalyst operation,the ethylene glycol yield is still stable.

Embodiment 7

The acid-resistant alloy main catalyst is Ni10Sm5Sn10Al9W90, and theamount added is 180 g.

The feed composition is: 60 wt % glucose, 2 wt % sodium tungstate, 38 wt% water, and the density of the sugar solution is about 1.29 g/cm³.

The reaction pressure is 12 MPa, and the reaction temperature is 260° C.

Other operating conditions are the same as in embodiment 2.

Results: the conversion rate of glucose is 100%; the diol yield is 75%,wherein the ethylene glycol yield is 65%, the propylene glycol yield is7%, and the butylene glycol yield is 3%; the methanol and ethanol yieldis 11%, and other yields are 14%.

Embodiment 8

The acid-resistant alloy main catalyst is Ni90Ce3Sn60Al9W20Mo20P0.01,and the amount added is 5 g.

The feed composition is: 5 wt % glucose, 0.05 wt % sodium tungstate,94.95 wt % water, and the density of the sugar solution is about 1.02g/cm³.

Reaction system pH=1.

The reaction pressure is 6 MPa, and the reaction temperature is 180° C.

Other operating conditions are the same as in embodiment 2.

Results: the conversion rate of glucose is 100%; the diol yield is 65%,wherein the ethylene glycol yield is 53%, the propylene glycol yield is9%, and the butylene glycol yield is 3%; the methanol and ethanol yieldis 4%, and other yields are 31%.

Embodiment 9

The acid-resistant alloy main catalyst is Ni80La1Ce0.5Sn30Al5; otheroperating conditions are the same as in embodiment 2.

Results are similar to those of embodiment 2.

Embodiment 10

The acid-resistant alloy main catalyst is Ni70Sm1Sn10Al7W5Mo0.5, and theamount added is 1500 g.

The feed composition is: 40 wt % glucose, 60 wt % water, 0.5 wt % sodiumtungstate, and the density of the sugar solution is about 1.18 g/cm³.

Reaction system pH=4.2.

Other operating conditions are the same as in embodiment 2.

Results: the conversion rate of glucose is 100%; the diol yield is 87%,wherein the ethylene glycol yield is 80%, the propylene glycol yield is5%, and the butylene glycol yield is 2%; the methanol and ethanol yieldis 3%, and other yields are 10%.

Clearly, the abovementioned embodiments of the present invention aremerely examples given to explain the present invention clearly, and byno means define the embodiments of the present invention. A personskilled in the art could make other changes or modifications indifferent forms on the basis of the explanation above. It is notpossible to list all embodiments here exhaustively. All obvious changesor modifications extended from the technical solution of the presentinvention shall still fall within the scope of protection of the presentinvention.

The invention claimed is:
 1. A method for preparing a diol comprisingcontacting a sugar and hydrogen with a catalyst in water in a reactor toprepare a diol; wherein the catalyst is a composite catalyst consistingof a main catalyst and a cocatalyst; the main catalyst is awater-insoluble acid-resistant alloy comprising nickel, one or more rareearth elements, tin and aluminum; and the cocatalyst is a solubletungstic acid salt and/or an insoluble tungsten compound.
 2. The methodof claim 1, wherein the diol is ethylene glycol.
 3. The method of claim2, wherein the pH of the reactor is 1-7.
 4. The method of claim 1,wherein the sugar is selected from the group consisting of one or morefive-carbon monosaccharides, disaccharides, and oligosaccharides;six-carbon monosaccharides, disaccharides and oligosaccharides; solublefive-carbon polysaccharides and soluble six-carbon polysaccharides. 5.The method of claim 1, wherein the sugar is provided in the form of anaqueous sugar solution, wherein the aqueous sugar solution has aconcentration of 5-60 wt %.
 6. The method of claim 5, wherein the amountof the soluble cocatalyst used is 0.01-5 wt % of the aqueous sugarsolution.
 7. The method of claim 6, wherein the amount of the insolublecocatalyst used is 0.5-50 wt % of the main catalyst.
 8. The method forpreparing a diol as claimed in claim 1, characterized in that theacid-resistant alloy comprises, in parts by weight, 10-90 parts nickel,1-5 parts rare earth element, 1-60 parts tin and 5-9 parts aluminum. 9.The method of claim 1, wherein the acid-resistant alloy comprises, inparts by weight, 10-90 parts nickel, 1-5 parts rare earth element, 1-60parts tin, 5-9 parts aluminum, 1-90 parts tungsten and 0.5-20 partsmolybdenum.
 10. The method of claim 1, wherein the soluble tungstic acidsalt is selected from the group consisting of ammonium tungstate, sodiumtungstate, sodium phosphotungstate and combinations thereof.
 11. Themethod of claim 1, wherein the insoluble tungsten compound is selectedfrom tungsten trioxide, tungstic acid and a combination thereof.
 12. Themethod of claim 1, wherein the amount of the main catalyst used is0.01-10 times the amount of sugar fed per hour.
 13. The method of claim1, wherein the reactor has a reaction pressure of 6-10 MPa, a reactiontemperature of 180-250° C., and a reaction time of 0.5-3 h.
 14. Themethod of claim 1, wherein the method is continuous.
 15. The method ofclaim 14, wherein the amount of main catalyst added is: 0.01-5 kg ofmain catalyst is added per 1000 kg of sugar fed.
 16. The method of claim1, further comprising separating the cocatalyst from the diol and thenrecycling the cocatylst in a subsequent method.
 17. The method of claim1, wherein the reactor is slurry bed reactor.